This invention relates to surface treatment of paper or paperboard, particularly additives applied to paper at the size press during paper manufacture. While the additives are generally referred to as paper sizes, their benefits are not strictly limited to imparting water resistance. They provide related benefits particularly enhanced printing by ink jet printers, photocopiers, and other means of reproducing images, commonly called "office printers".
Chemicals are added to paper at the dry end of the process of paper making by means of a size press or calendar boxes. These may be applied to paper that will subsequently be coated with a mineral coating, but are most important to performance of grades of paper that have no subsequent coating, because in those cases, the size pressing or calendar box treatments are the last treatments and must completely prepare the surface of the paper for its end uses. The most common additive applied at the dry end is starch, as a water solution of cooked starch, which may be from many vegetable sources, most commonly waxy maize or dent corn. A second important additive is often a resinous or polymeric material which imparts water resistance to the paper in addition to that imparted by wet end additives. This surface size is important in controlling and altering the surface of the paper so it is receptive to any of the many printing methods. It is particularly important to achieving a surface receptive to high resolution office printing such as ink jet and laser printer printing.
For many years the papermaker has had only a few types of size press sizes from which to choose. Ammonium salts of half esters of maleic acid copolymerized with styrene and salts of acrylic acid polymerized with styrene are two types commonly used. Emulsions of polyesters and polyurethanes are also sometimes used. These sizes have several general drawbacks to their use and each has specific detriments to the papermaking process. Some have negative features in the effect they have on the surface of the sheet. Styrene-based polymers are known to produce foam in the process equipment which creates deposit problems and may contribute to poor or erratic performance as a surface modifier. This is a serous deficiency in that chemical counter-measures are often self defeating: defoamers and anti-foams are often composed of surface active materials which cause the paper surface to be more easily wetted. Styrene-maleic and other film forming polymers are also reported to impede drying. Emulsions of polyesters or urethanes are easily destabilized by the high temperatures, hydrodynamic shear and conditions favoring evaporation usually found in starch-holding tanks. Efficiency of such emulsion sizes is generally conceded to decrease if the dispersed particles coagulate. More commonly, the ultimate particle size is quite large and the efficiency of the material as a surface treatment is low. Conversely, small diameter particles require large amounts of stabilizing agents which are usually wetting agents and foaming agents.
Solution polymers are more effective as sizing agents the higher their molecular weight. The most effective water soluble surface sizing polymers, therefore, form very viscous solutions. For this reason they are usually sold as dilute solutions of 10% top 20% solids content. The water present contributes substantially to the delivered cost of such polymers, thus a lower viscosity and higher solids would be desirable. High molecular weight also contributes to the foaming tendency of such polymers, and to the difficulty in removing deposits of such chemicals when they form in the papermaking system, at the high liquid level of size press feed tanks for example.